Methylmercury Production and Degradation Under Light and Dark Conditions in the Water Column of the Hells Canyon Reservoirs, USA.

Methylmercury (MeHg) is a highly toxic form of mercury (Hg) that can bioaccumulate in fish tissue. Methylmercury is produced by anaerobic bacteria, many of which are also capable of MeHg degradation. In addition, demethylation in surface waters can occurs via abiotic sunlight-mediated processes. The goal of this study was to understand the relative importance of microbial Hg methylation/demethylation and abiotic photodemethylation that govern the mass of MeHg within an aquatic system. The study location was the Hells Canyon Complex (HCC) of three reservoirs on the Idaho/Oregon border, USA that has fish consumption advisories due to elevated MeHg concentrations. Our study utilized stable isotope addition experiments to trace MeHg formation and degradation within the water-column of the reservoirs to understand the relative importance of these processes on the mass of MeHg using the Water Quality Analysis Simulation Program (WASP). The results showed that rates of MeHg production and degradation within the water column were relatively low (<0.07 d-1 ) but were sufficient to account for most of the MeHg observed with the system. Most MeHg production within the water column appeared to occur in the spring when much of the water-column was in the processes of becoming anoxic. In the surface waters, rates of photodemethylation were relatively large (up to -0.25 d-1 ), but quickly decreased at depths deeper than 0.5m below the surface. These results can be used to identify the relative importance of MeHg processes that can help guide reservoir management decisions. This article is protected by copyright. All rights reserved.